A device such as a superconducting device that utilizes characteristics thereof at a low temperature is mounted in a refrigeration unit that is formed with a refrigerating machine for generating a low temperature and a heat-insulating vacuum chamber for restraining heat from flowing into a low-temperature portion. In reducing the load on the refrigerating machine in the refrigeration unit, the heat-insulating vacuum chamber plays an important role. Conventionally, in a system that includes a sealed-type heat-insulating vacuum chamber not having an evacuation pump for discharging unnecessary gas to the outside, a non-evaporation-type getter is provided in the heat-insulating vacuum chamber so as to maintain the heat-insulating vacuum chamber. When the degree of vacuum in the heat-insulating vacuum chamber is degraded, the non-evaporation-type getter is activated to improve the degree of vacuum. Such a system has been conventionally used.
In that system, however, the number of input/output ports for drawing signal lines from the inside of the heat-insulating vacuum chamber to the outside is large. When the amount of air leakage from the input/output ports increases, the adsorptive removal capacity of the non-evaporation getter is saturated, and the heat-insulating vacuum chamber cannot be maintained over a long period of time.
Also, in a case where the low-temperature portion of the refrigeration unit is used at a temperature in the neighborhood of 70 K as in a superconducting device including a cuprate superconductor, nitrogen leaking from the atmosphere into the heat-insulating vacuum chamber repeatedly goes through the following steps: (1) the nitrogen is liquefied upon contact with the low-temperature portion, and heat of aggregation is supplied to the low-temperature portion; (2) the liquefied nitrogen drops onto a normal-temperature portion; and (3) the liquefied nitrogen in contact with the normal-temperature portion is gasified. As steps (1) through (3) are repeated, heat is supplied from the normal-temperature portion to the low-temperature portion, and the load on the refrigerating machine becomes larger. This leads to failures in operation of the superconducting device that requires precise temperature control.